Exhaled breath diagnostics as a clinical method is reaching more and more acceptance in a variety of different diseases. To ensure reproducible measurement results, exhaled breath from patients should conform to certain protocols dictating the physical parameters (e.g. flow rate, pressure, temperature etc.) under which the test should be made. Thus it is sought to perform measurements under substantially constant flow of exhaled breath, despite variations in applied pressure.
In WO 2006/080885 A1 a constant flow regulator device for maintaining a constant flow of fluid is disclosed. The device comprises an inlet duct for incoming fluid, a housing, and a movable partition facing the inlet duct and being subjected to an elastic force. A fluid passage of variable cross section area is formed between the inlet duct and the movable partition. The housing and movable partition form an inner compartment in fluid communication with the inlet duct for establishing a fluid pressure inside the inner compartment approximately equal to the fluid pressure in the inlet duct. The size of the movable partition is significantly greater than the size of the inlet duct such that, in use, the partition is moved towards the inlet duct against the elastic force when the fluid pressure in the inlet duct increases to reduce said fluid passage cross section area, and vice versa, thereby maintaining constant fluid flow.
The current trend towards handheld point-of-care devices has brought a need for miniaturized flow handling systems. The previously disclosed device requires a significant size of the partition. It is therefore desired to find alternative solutions to the problem of regulating a fluid flow that may be suitable for miniaturisation, and that still may be used to regulate the relatively large flows of exhaled breath. For example, in asthma monitoring (fractional exhaled NO, FENO), regulatory guidelines dictates that measurements of nitric oxide concentration should be made at an exhaled flow rate of 50±5 ml/s (ATS/ERS recommendations for standardized procedures for the online and offline measurement of exhaled lower respiratory oxide and nasal nitric oxide, 2005. Am J Respir Crit Care Med 2005; 171:912-930).